What is causing disabled options when adding a mesh/curve object to the scene?
It sounds like you may have disabled Global Undo
That panel is called the redo panel, because when you change a setting on it, blender actually redoes the previous operator with the new settings.
If blender can't undo the previous operator first, running it again will add it's effect on top of the result of the previous operator (which is not the desired effect at all).
So this panel is disabled when undo is not available.
For 2.8x and 2.9.
Edit &gt; User preferences &gt; System &gt; Memory &amp; Limits: Global Undo


For 2.7X use
CtrlAltU&gt; User preferences &gt; Editing: